Filaggrin is a major component of the mammalian epidermis and is synthesized in the granular layer as large polyprotein, precursor (termed profilaggrin). Profilaggrin consists of many (up to 20 or more) identical filaggrin repeats fused to a calcium binding domain and initially accumulates as amorphous, non-membrane bound, intracellular deposits termed keratohyalin granules. Subsequently, profilaggrin is acted upon by a series of processing enzymes liberating the smaller filaggrin repeats. These are rapidly dispersed from the granules to freely interact with the keratin filament network of these cells. It is believed that electrostatic interactions between filaggrin and keratin results in a tighter, more dense filament network. Later, filaggrin undergoes further post translational modifications and is finally degraded to individual amino acids. These are thought to contribute to the moisture content of the skin and the conversion of one of these amino acids, histidine to urocanic acid, is thought to offer protection against U.V. light. Given the abundance of filaggrin in epidermal keratinocytes and its putative roles in epidermal biology, any perturbation in its biosynthesis could be expected to produce adverse consequences. It is noteworthy then, that several diseases have been identified where filaggrin expression or processing appears perturbed, such as in the ichthyoses and psoriasis. One approach to investigate the function of a particular protein is to overexpress a mutant form of it in transgenic mice. However, since profilaggrin consists of 10-20 identical repeating protein subunits, it is not clear that a dominant mutation could be readily constructed. Moreover, the paucity of knowledge concerning which amino acids are critical for profilaggrin biosynthesis and function further limits our ability to design dominant mutations. To circumvent these problems, it is proposed to investigate filaggrin function by ablating or inactivating the profilaggrin gene from the germ line of mice using homologous recombination. In addition to determining the contribution of filaggrin to such diverse roles as keratin filament packing and moisture retention, mice lacking a functional profilaggrin gene may exhibit a phenotype that resembles a human disease. Such a candidate disease is ichthyosis vulgaris, in which it has been observed that profilaggrin expression is low or absent. These knockout studies will determine whether filaggrin is in fact the site of the genetic insult in this or in other related skin diseases.